Reduction of false alarms in electronic theft detection systems

ABSTRACT

Electronic theft detection systems are disclosed which have a particular frequency within which protected articles cause electronic response. False alarms are reduced by detecting similar electronic responses outside the frequency range and temporarily deactivating the system when such responses are detected. In a swept frequency system deactivation signals are produced during the portion of the frequency sweep outside the particular frequency and the deactivation remains during the remaining portion of the frequency sweep past the particular frequency.

O I United States, Patent 1191 1111 3,868,669

Minasy 1 51 Feb. 25, 1975 [54] REDUCTION OF FALSE ALARMS IN 3,711,8481/1973 Martens 340/258 C ELECTRONIC THEFT DETECTION 3,781,860 12/1973Freyling, Jr... 340/280 3,801,977 4/1974 Cotter 340/258 A SYSTEMS [75]Inventor: Arthur J. Minasy, Woodbury, NY. Primary Examiner Glen Swarm "I[73] Assignee: Knogo Corporation, Westbury, N.Y. Attorney 8Firm-Fi1ZPatfik, Celia, Harper & Sc'nt 22 Filed: Apr. 13, 1973 l 0 [21]Appl. No.: 351,019 57 ABSTRACT Electronic theft detection systems aredisclosed which [52] US. Cl. 340/280, 340/258 C h a particular frequencywithin which protected [5 Illlt. Cl. articles cause electronic responseFalse alarms are re- Fleld of Search 340/258 C, 253 A; duced bydetecting similar electronic responses out- 343/5 325/478, side thefrequency range and temporarily deactivating the system when suchresponses are detected. In a References Cited swept frequency systemdeactivation signals are pro- UNITED STATES PATENTS duced during theportion of the frequency sweep out- 2,794,974 6/1957 Bagno et a1 340/258A Side the Particular frequency and the deactivation 3,218,556 11/1965Chisholm 325/478 mains uring the remaining portion of the frequency3,465,336 9/1969 Fishbein et a1. 340/258 A sweep past the particularfrequency. 3,577,136 5/1971 Wolf 340/280 3,696,379 10/1972 Minasy340/280 11 Claims, 4 Drawmg Figures S 55F I 7ZIYEAGL Mm Gayggnnxqfe/rwlr Awa /m 0 500 A? Own/1704 82 72 0 T pe'ccn/tg i fl 75/750 M/XLQIF/EIE 727 12 Gem SHIICH/IYG flue/e 450- 007 /9 wk; do l c r l moor/15:

I I flaw/spam? 72. 1 1 I I l %\-/L l I\ F 76 A686 Gnzz SIGN/7L Gm:SIG/#14007 0V4) '74 i 59%;; firs/1m Q0 0V4) Due/MG Abv/dvae a|6 NG Ila/4FSermvOFrfi/Efi 94 9b fine/vanil a? SIG/ML susmm'ma C/pcu/T 77M: Comm/1rC/iqcu/r Amen REDUCTION OF FALSE ALARMS IN ELECTRONIC THEFT DETECTIONSYSTEMS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to the control of theft detection systems and moreparticularly it concerns arrangements for reducing the occurrence offalse alarms from such systems due to extraneous and sporadic electricaleffects, such as may result from the operation of electrical machinerynear the system.

2. Description of the Prior Art The present invention is especiallysuitable for use in conjunction with electronic theft detection systemsof the type described in U.S. Pat. Nos. 3,493,955 and 3,500,373. In bothsystems, each of the articles to be protected from theft has aelectronic responder circuit attached to it. This circuit may beconcealed in a wafer like element which may also serve as a price labelor the like for the protected article. The articles are maintained in anenclosure having limited egress and checkpoints are set up at eachegress. A transmitter is provided at the checkpoint to transmit aninterrogation signal and receiver means are provided to note anyresponse produced by the interaction of a wafer responder circuit withthe transmitted signal field in the vicinity of the checkpoint. In thecase of the systems described in U.S. Pat. No. 3,493,955, the waferresponder circuits respond to the transmitted interrogation signal,which is at a first frequency, to produce a response signal at a secondfrequency. The receiver means are tuned to detect this second frequency.

In the case of the system described in U.S. Pat. No. 3,500,373, thewafer responder circuits are resonant circuits tuned to resonate at thetransmitted interrogation frequency. When these wafer responder circuitsare brought into the transmitted interrogation signal field they absorbsome of the transmitted energy. The receiver means monitorsthetransmitted signal, which changes in amplitude due to thisabsorbtion. In order to maximize sensitivity the transmitter of thissystem produces an output frequency which sweeps cyclically over a givenrange which includes the resonant .frequency of the wafer respondercircuits. This causes a series of responses in the form of impulseswhich occur at a repetition rate corresponding to the frequency sweeprate.

While in both of the above described systems the wafer circuit serves toproduce a unique electrical response to the interrogation signal, thepossibility exists that similar responses may be produced by nearbyelectrical equipment such as switches, motors, relays etc., or by otherextraneous or sporadic electrical effects; and in such case a falsealarm signal may result.

In another U.S. Pat., No. 3,696,379, there is described one means forcounteracting the effects of nearly electrical equipment and extraneousor sporadic electrical effects. According to this patent, a separatereceiving system is provided near, but not at, the checkpoint. Thisreceiving system, because of its location, will not respond to theeffects of wafer responder circuits passing through the checkpoint.However, it does respond to the signals of nearby electrical machinery,etc., which might produce false alarms. Whenever such signals aredetected by the separate receiving systern, it operates to deactivatethe main receiving system so that the false alarm cannot be produced.

SUMMARY OF THE INVENTION The present invention provides an alternativeway to reduce the occurrence of false alarm signals. According to thepresent invention a separate receiving means is provided; but instead ofbeing responsive to the same frequency signals as the main receivingsystem but at a different location, the separate receiving system of thepresent invention responds to different frequency signals at the samelocation as the main receiving system.

According to one embodiment of the present invention the separatereceiving means comprises a receiving channel tuned to respond toelectrical effects which occur only within a frequency range which doesnot include the response signal frequencies. Should those effects exceeda predetermined magnitude, the separate receiving channel produces anoutput which is utilized to deactivate the alarm.

According to a further embodiment of the invention, the transmitter andthe wafer responder circuits are arranged so that the wafer respondercircuit produces a series of spaced responses. A gating system is set upin a manner which on one hand permits the main receiving system tofunction only at the times the spaced responses would occur and which onthe other hand permits the separate receiving system to function duringthe intervening times. Should the output of the main receiving systemexceed a predetermined level, it will activate an alarm. However, shouldthe output of the separate receiving system exceed a predeterminedamount it will override the effect of the first receiving system anddeactivate the alarm.

In the preferred embodiment of the invention a transmitting system isarranged to transmit an interrogation signal at a frequency which isswept back and forth at a given rate and over a range which includes apredetermined wafer responder circuit response frequency. The waferresponder circuits on the protected articles resonate at the responsefrequency and change the impedance in the vicinity of the transmittingsystem during the times that the transmitted frequency sweeps past theresponse frequency. Thus when a wafer responder circuit passes through acheck-point in the transmitter antenna field, it causes a series ofresponses to be produced at a repetition rate corresponding to thetransmitter frequency sweep rate. In order to protect the system frompossible false alarms a frequency selective switch is provided. Thisfrequency selective switch is arranged to direct the receiver responseinto a first or a second channel referred to respectively as a noisechannel and a signal channel. The frequency selective switch is tuned tospan the wafer responder circuit resonant frequency range and isarranged to open the signal channel when the transmitter frequencypasses through that range. Whenever the response level exceeds apredetermined threshold in the signal channel an alarm is activated. Thefilter switch also opens the noise channel during the remaining portionsof each transmitter frequency sweep. Whenever the signal level in thenoise channel exceeds a given threshold a deactivation signal isproduced which deactivates the alarm for a given length of time.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto. Thoseskilled in the art will appreciate that the conception upon which thisdisclosure is based may readily be utilized as a basis for the designingof other structures and methods for carrying out the several purposes ofthe invention. It is important, therefore, that the claims be regardedas including such equivalent constructions and methods as do not departfrom the spirit and scope of the invention.

BRIEF. DESCRIPTION OF THE DRAWINGS Specific embodiments of the inventionhave been chosen for purposes of illustration and description, and .areshown in the accompanying drawings forming a part of the, specification,wherein:

FIG. 1 is a fragmentary perspective-view illustrating a checkpoiritforarticle theft detection in which the present invention is embodies;

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown adoorway separating an enclosure 12 .from an exterior region 14. Thedoorway lflconstitutes one of a limited number of egress passagewaysfrom the enclosure. 12; and accordingly it serves as a checkpoint forascertaining any unauthorized removal of articles from the enclosure.The doorway 10 is provided with several antenna windings 16', I8 and 20arranged such that when the windings are energized from a transmitter22, they establish electromagnetic fields'in the region of the doorway10. These electromagnetic fields serve as a monitoring means since anyobject passing through the doorway 10 must also pass through theelectromagnetic field. All articles to be protected are provided with awafer 24 (shown in dotted outline) which encapsulates an electronicresponder circuit. When the wafer is brought through the doorway 10, itscircuit reacts with the electromagnetic field. This response is detectedby a receiving system (not shown which activates an alarm. If anarticle. has

been purchased special means'may be employed for removal or-deactivationof the wafer circuit so that it may pass through the electromagneticfield without causing an alarm actuating disturbance.

FIG. 2 illustrates in block diagram form, the application of thepresent-invention to a rebroadcaster type electronic theft detectionsystem such as that shown and described 'inU.S. Pat. No. 3,493,955. Ascan be seen in FIG. 2, there is provided a transmitter 26 whichenergizes a transmitter antenna 28 so that the transmitter antennacontinuously emits an electromagnetic field at'a monitoring frequencyof, for example, 27.2 megahertz (MHZ). The transmitter antenna 28 maycomprise one or more of the windings 16,18 and 20 (FIG. 1) so that theelectromagnetic field fills the region of the doorway 10. A respondercircuit 30, which may be encapsulated in the wafer 24 of FIG. I, isconfigured to respond to the 27.2 MHZ field and to emit a responsesignal at a frequency of between 5.1 and 5.9 MHZ, for example.

The responder circuit output signals are intercepted by a receiverantenna 32; and they are detected and amplified in a receiver 34. Thereceiver antenna 34 may also comprise a winding at the doorway 10 (FIG.1). The output of the receiver 34 passes through a broad band amplifier36; and from there the detected signals pass'along two channels. Thefirst channel includes a signal filter 38 which is relatively sharplytuned to the output frequency of the responder circuit 30. The output ofthe signal filter 38 passes to a threshold detector 40 which, uponreceipt of a signal of a predetermined magnitude, generates an alarmactuation signal. The alarm actuation signal from the threshold detectorpasses through a normally open gate circuit 42 to actuate an alarm 44.The alarm 44 may be any audio or visual type indicator means well knownin the art.

The second channel of the receiver system includes a noise filter 46which is tuned to pass signals whose frequency components do not liewithin the selected signal range, i.e., between 5.1 and 5.9 MHZ. Theoutput of the noise filter is applied to a threshold detector 38. Thethreshold detector is set to respond to a predetermined noise level and,upon the occurrence of such noise level, to apply a signal to an inhibitterminal 50 of the normally open gate circuit 42.

In operation of the system of FIG. 2, articles to be protected areprovided with wafers 24 (FIG. I) which contain responder circuits 30capable of emitting an electromagnetic signal of between 5.1 and 5.9 MHZwhen energized by an electromagnetic field at 27.2 MHZ. Thus, whenever aprotected article containingan operative responder circuit 30 passesthrough the doorway 10 (FIG. 1) and through the field of the transmitterantenna 28, it responds to the continuously transmitted electromagneticfield in the doorway and in turn energizes-the receiver antenna'32.;Signals from the receiver antenna 32are detectedlbyrthe receiver 34and arepassed through the broad band amplifier 36', the filter 38, thethreshold detector 40, and the normally open gate circuit 42, toactivate the alarm 34.

Should there occur any sporadic or extraneous electrical disturbancescapable of causing premature or improper actuation of the alarm 44, suchdisturbances would, according to the present invention, be detected andutilized in a novel way to protect against any premature orimproperalarm actuation. The present invention is based inpart on thediscovery that sporadic and extraneous electrical disturbancesoccur overa rather broad frequency band substantially greater than the selectedresponse signal frequency. Thus, in the present case, while sporadicelectrical disturbances may produce electromagnetic fields inthevicinity of the doorway), within the 5.l to 5.9 MHZ response frequencyrange, those same disturbances will also produce other electromagneticfulls at frequencies both above and below the response frequency range.These other electromagnetic fields are detected by the receiver 34; andwhile the receiver signals they produce do not pass through the signalfilter 38, they do pass through the noise filter 46. These signals passthrough the threshold detector 48 and appear at the inhibit terminal ofthe gate circuit 42. This closes the gate circuit 42 and prevents thesporadic electrical disturbance induced signals in the signal channelfrom passing through the channel to activate the alarm 44. When theelectrical disturbance ceases, the signals in the noise channelterminate, and the inhibit terminal 50 of the gate circuit 42 isdeactivated, thus allowing the gate circuit to revert to its normallyopen condition. Thus, any responder circuit induced signals which arenot accompanied by signals outside the selected response fre quencyrange may then pass through the signal channel and through the gate 42to activate the alarm 44. The noise filter 46 must, of course, also betuned to prevent passage of signals at the transmitted frequency inorder to prevent continuous indications of false alarm from energyreceived directly from the transmitter.

FIG. 3 shows in block diagram form, a modified version of the presentinvention as applied to an electronic theft detection system of the typeshown and described in U.S. Pat. No. 3,500,373. As shown in FIG. 3, thetheft detection system includes sweep generator 60 capable of producinga repetitive output sweep, for example a sine wave voltage whoseamplitude varies at a frequency of 300 cycles per second. The sweepgenerator output is applied to a tunable transmitter oscillator 62which, in response to the signals from the sweep generator 60, producesan output frequency which shifts, for example, from 1.95 to 2.05 MHZ ata 300 cycle per second rate. This varying frequency output is applied toa transmitter amplifier 64 where it is amplified and then applied to atransmitter antenna 66. This transmitter antenna may comprise one ormore of the windings 16, 18 and 20 arranged in the doorway I0 of FIG. 1.The antenna 66 produces an electromagnetic field in the vicinity of thedoorway which varies in frequency between I95 and 2.05, MHZ at a 300cycle per second rate. A responder 68, which may be encapsulated withinthe wafer 24 of FIG. 1, comprises a resonant circuit, such as shown inthe aforementioned U.S. Pat. No. 3,500,373. This circuit is tuned toresonante at some fixed frequency between 1,95 and 2.05 MHZ. Morespecifically, in the present embodiment, the responder circuit is tunedto resonate at 2.00 MHZ.

A receiver 70 is connected to a point between the transmitter amplifier64 and the antenna 66. The output of the receiver 70 is applied to athreshold detector 72, and its output in turn is connected in parallelto a noise gate circuit 74 and to a signal gate circuit 76. The outputof the signal gate circuit 76 is connected through a time constantcircuit 78 to an alarm 80.

The circuit of FIG. 3, as thus far described, operates to detect thepresence of the responder circuit 68 in the field of the antenna 66,i.e., in the vicinity of the doorwayl0 (FIG. 1), in the manner describedin aforementioned U.S. Pat. No. 3,500,373. Thus, the vicinity of thedoorway is filled with a high frequency electromagnetic field whosefrequency is swept continuously at a 300 cycle per second rate back andforth over a frequency range which includes the resonant frequency ofthe responder circuit 68. This frequency sweep is illustrated by a curve(a) in FIG. 4. The resonant frequency of the responder circuits 68,which is indicated by a cross hatched strip (b), is chosen to be nearthe middle of the frequency range. The width of the strip (b) dependsupon the Q of the resonant circuits 68, that is, upon the sharpness withwhich they can be tuned.

Each time the antenna field frequency sweeps across the resonantfrequency of a resonant circuit 68 which is in its field, i.e., which ispresent in the doorway 10 of FIG. I, the impedance presented to theantenna 66 decreases and a greater amount of energy flows out theantenna. Because of this, the amount of transmitter energy presented tothe receiver 70 decreases. This phenomenon is experienced twice duringeach cycle of transmitter frequency sweep, i.e., at 600 times persecond. The receiver circuits are designed, as described in U.S. Pat.No. 3,500,373, to respond to the occurrence of energy decreases incidenton the receiver at 600 times per second, and to supply a signal to thethreshold detector 72 when this occurs. Signals from the thresholddetector 72 pass through the signal gate circuit 76, the time constantcircuit 78 and activate the alarm 80.

The remainder of the system of FIG. 3 is designed to disable theactuation of the alarm 80 whenever spurious electromagnetic orelectrical disturbances occur, which otherwise might be interpreted bythe receiver 70 and the threshold detector 72 as a responder circuit 68passing through the field of the antenna 66. The disabling portion ofthe system of FIG. 3 includes a sharply tuned oscillator 82 which istuned to a frequency of 1.5 MHZ. The output of this oscillator is applied to a mixer 84 along with output signals from the tunabletransmitter oscillator 62. These signals, when mixed in the mixer 84,produce an output signal which varies between 450 and 550 KHZ(kilohertz) at a 300 cycles per second rate. The mixer output is appliedto i output terminal 92 which are alternately energized.

Normally, the noise gate actuation output terminal 92 of the monostablemultivibrator 88 is energized. However, whenever the output of the mixer84 reaches a frequency approaching a frequency corresponding to that ofthe responder circuit 68, the switching filter 86 produces an outputcausing the monostable multivibrator 88 to deenergize the noise gateactuation terminal 92 and to energize the signal gate actuation terminal90. Since the switching filter 86 is tuned to have an equivalent bandpass range (0) which is slightly greater than the responder circuitresponse frequency range (b), the switching filter 86 will operate totrigger the monostable multivibrator whenever the antenna frequencyapproaches the responder circuit resonant frequency in both directionsof the antenna frequency sweep, irrespective of whether the antennafrequency is increasing or decreasing. The monostable multivibrator 88reverts back to its normal state, i.e., with the noise gate actuationoutput terminal 92 energized and the signal gate actuation outputterminal 90 deenergized, after a predetermined length of time followingeach output from the switching filter 86. This predetermined length oftime is slightly longer than the length of time required for the antennafrequency sweep to circuit 68. That is, the monostable multivibrator 88remains energized over a period of time which straddles the periodduring which the antenna frequency sweeps across the resonant frequencyof the'responder circuits 68. This energization of the monostablemultivibrator 88 is illustrated by a curve (d) in FIG. 4.

It will be noted from FIG. 4 that the monostable multivibrator 88 isswitched to its non-stable state at different transmitter frequenciesdepending upon whether the transmitter frequency is increasing ordecreasing. This is made possible by thetuning of the switch filter 86.This tuning is broader than that of the responder circuits 68; and itproduces a switching signal at the multivibrator 88 just before theantenna frequency reaches the resonant frequency of the respondercircuits 68.

' gate circuit 74 is connected through a signal sustaining circuit 94 toan inhibit terminal 96 of the signal gate circuit 76. The signalsustaining circuit 94 is constructed to cause outputs from the noisegate circuit 74 to remain on the inhibit terminal 96 of the signal gatecircuit 76 for a predetermined length of time (e.g., 0.1 seconds), whichis substantially in excess of the frequency sweep period of thetransmitter. This allows time for any spurious interference to terminatebefore the system reverts to normal operation.

The operation of the system of FIG. 3 will now be described. The tunabletransmitter oscillator 62 energizes the antenna 66 so that anelectromagnetic field is emitted from the antenna 66 into the vicinityof the doorway 10 (FIG. 1). The frequency of this electromagnetic fieldis continuously varied by the action of the sweep generator 60 so thatthe antenna field undergoes successivefrequencysweeps between 1.95 and2.05 MHZ circuit 68 passes through the doorway 10 (FIG. 1) andencounters the field of the antenna 66, the responder circuit willresonate each time the frequency of the antenna field passes through2.00 MHZ. Since this happens twice during each frequency sweep, aresonant response is generated at the rate of 600 responses per second.As indicated previously, the nature of these resonant responses is suchthat they produce a decrease in antenna output impedance and acorresponding reduction of energy applied to the receiver 70. Thesedecreases in energy are detected by the receiver 70; and its filterconfiguration is such as to select those ener'gy decreases which occurat the 600 responses per second rate. When this occurs, the receiver 70produces an output which is applied to the threshold detector 72 and itsoutput in turn is applied to the noise gate circuit and the signal gatecircuit 74 and 76 respectively. In the event that the output from thereceiver 70 occurs while the anetnna 66 is emitting a frequency withinthe range (0) of FIG. 4, i.e., corresponding substantially to theresonant range (b) of the responder circuits 68; then the switchingfilter 86 will have caused the monostable at a 300 cyclespersecondlrate'. Whenever a responder multivibrator 88 to energize itssignal gate actuation output terminal 90 to open the signal gate circuit76 and allow the-output from the threshold detector 72 to pass throughthe timevconstant circuit 78- to energize the alarm 80 for predeterminedlength of time.

On the other hand, if the receiver causes an output to pass through thethreshold detector 72, when the antenna 66 is transmitting outside therange (c) of FIG. 4, then the switching filter will not have caused themonostable multivibrator 88 to energize its signal gate actuation outputterminal 90. Instead, the noise gate actuation output terminal 92 of themonstable multivibrator 88 remains energized. Accordingly, the-signalgate 76 remains closed; and the receiver output, which passes throughthe threshold detector 72, is stopped at the signal gate circuit 76 anddoes not actuate the alarm 80. Thus, even though a resonant circuit maybe present in the antenna field, it will not cause a false alarm if itproduces any resonant response outside the preselected resonantfrequency range of the responder circuits 68.

The system of FIG. 3 provides additional protection in that it usesinformation obtained during the portion of the frequency sweep outsidethe preselected responder circuit resonant frequency range to controlits operation when the transmitter frequency subsequently reaches thepreselected resonant frequency of the responder circuits. Thus, shouldresponses be detected outside the responder circuit resonant frequencyrange, the system will, for a predetermined length of time, prevent anyalarm actuation even from subsequent responses which do occur within theresonant frequency range. This protects against false alarms fromobjects which pass through the antenna field and which have anassortment of electrical characteristics producing resonant responses inseveral frequency ranges including the resonant range of the respondercircuits 68.

the noise gate circuit 74 in an open condition. As a re- ,sult of this,the noise gate cir'cuit-74 applies a signal through the signalsustaining circuit 94 to the inhibit terminal 96 of the signal gatecircuit 76. Accordingly, as the antenna frequency continues to sweep andultimately passes through the response ranges (b) and (c) the resultingswitching of the monostable multivibrator 88 which. opens the signalgate circuit 76 is rendered ineffective because of thecontinued'presence of an inhibit signal at the'inhibit terminals 96 ofthe gate circuit 76. This maintains the signal gate circuit 76 is closedfor a predetermined length of time (e.g., 0.1 seconds), which is well isexcess of the transmitter frequency sweep period. In most instances thesporadic electrical disturbance will have terminated. On the other hand,if the sporadic electrical disturbance should continue. the noise gatecircuit 74 will be allowed to produce an inhibit signal gate circuit 76on the next subsequent transmitter sweep.

It will be appreciated from the foregoing that the present inventionprovides protection from false alarms based upon active detection infrequency ranges out- 9 side the frequency range of the variousresponder devices.

Having thus described the invention with particular reference to thepreferred forms thereof, it will be obvious to those skilled in the artto which the invention pertains, after understanding the invention, thatvarious changes and modifications may be made therein without departingfrom the spirit and scope of the invention, as defined by the claimsappended hereto.

What is claimed is:

l. A method for reducing the occurrence of false alarms in an electronictheft detection system of the type wherein an interrogation signal isvaried infrequency and wherein predetermined changes in theelectromagnetic field in the vicinity of a checkpoint are produced bythe passage therethrough of protected articles carrying specialelectronic circuits whenever said interrogation signal passes through agiven frequency range, and wherein an alarm indication is produced inresponse to said predetermined changes, said method comprising the stepsof monitoring the interrogation signal frequency and preventing alarmindications when said interrogation signal frequency is outside saidgiven range.

2. A method for reducing the occurrence of false alarms in an electronictheft detection system of the type wherein an interrogation signal isvaried in frequency and wherein predetermined changes in theelectromagnetic field in the vicinity ofa checkpoint are produced by thepassage therethrough of protected articles carrying special. electroniccircuits whenever said interrogation signal passes through a givenfrequency range, and wherein an alarm indication is produced in responseto said predetermined changes, said method comprising the steps ofmonitoring for similar changes in the electromagnetic field in thevicinity of said checkpoint which occur while said interrogation signalis outside said given range and preventing alarm indications in responseto detection of said similar changes when said interrogation signalsubsequently passes through said given range.

3. A system for detecting the unauthorized passage of articles through acheckpoint, said system comprising a transmitter operative to transmitan interrogation signal which varies in frequency, responder circuitsmounted on articles whose unauthorized passage is to be detected, saidresponder circuits being operative to produce a predetermined change inthe electromagnetic field conditions in the vicinity of said checkpointin response to the incidence on said circuits of said interrogationsignal within a given frequency range, receiver means operative inresponse to said predetermined change to produce an alarm actuatingsignal, alarm means responsive to said actuating signal to produce analarm indication and means operative to prevent alarm actuation whilesaid interrogation signal is outside said given frequency range.

4. A system according to claim 3 wherein said responder circuits includeenergy absorbing resonant circuits and wherein said receiver means isconnected to respond to energy level changes in the vicinity of saidcheckpoint caused by the absorption of power in a responder circuitpassing through the checkpoint.

5. A system according to claim 3 wherein said means operative to preventalarm actuation comprises a gate circuit associated with said receiverand arranged in the path of said alarm actuating signal, a frequencysensitive switch connected to receive transmitted interrogation signalsand to produce switching signals to open said gate circuit when saidinterrogation signals are within said given frequency range and to closesaid gate circuit where said interrogation signals are outside saidgiven frequency range.

6. A system for detecting the unauthorized passage of articles through acheckpoint, said system comprising a transmitter operative to transmitan interrogation signal which varies in frequency, responder circuitsmounted on articles whose unauthorized passage is to be detected, saidresponder circuits being operative to produce a predetermined change inthe electromagnetic field conditions in the vicinity of said checkpointin response to the incidence on said circuits of said interrogationsignal within a given frequency range, receiver means operative inresponse to said predetermined change to produce an alarm actuatingsignal, alarm means responsive to said actuating signal to produce analarm indication and means associated with said receiver means forproducing alarm inhibit signals in response to said predetermined changewhich occurs while said interrogation signal is outside said givenfrequency range and means responsive to the occurance of said alarminhibit signals for inhibiting alarm indications when said interrogationsignal subsequently passes through said given frequency range.

7. A system according to claim 6 wherein said responder circuits includeenergy absorbing resonant circuits and wherein said receiver means isconnected to respond to energy level changes in the vicinity of saidcheckpoint caused by the absorption of power in a responder circuitpassing through the checkpoint.

8. A system according to claim 6 wherein said means associated with saidreceiver means comprises first and second gate circuits each connectedto receive signals from said receiver means, one of said gate circuitsbeing connected to permit signals passing therethrough to produce analarm indication, the other gate circuit beingconnected to permitsignals passing therethrough to override any opening of said, one gatecircuit for a predetermined length of time and a frequency sensitiveswitch connected to receive transmitted interrogation signals and toproduce switching signals to open said one gate when said interrogationsignals are within said given frequency range and to open said othergate when said interrogation signals are outside saidgiven frequencyrange. V

9. A system according to claim 8 wherein said frequency sensitive switchincludes a multivibrator having two output terminals which arealternately energized and which are connected, respectively, to controlterminals of said first and second gate circuits.

, 10. A system according to claim 8 wherein a signal sustaining circuitis connected to the output of said other gate circuit.

11. A system according to claim 10 wherein said transmitter is operativeto vary the frequency of said interrogation signal cyclically at a givenrate and wherein said signal sustaining circuit has a duration in excessof the cyclic period of frequency variation of said interrogationsignal. 7

1. A method for reducing the occurrence of false alarms in an electronictheft detection system of the type wherein an interrogation signal isvaried in frequency and wherein predetermined changes in theelectromagnetic field in the vicinity of a checkpoint are produced bythe passage therethrough of protected articles carrying specialelectronic circuits whenever said interrogation signal passes through agiven frequency range, and wherein an alarm indication is produced inresponse to said predetermined changes, said method comprising the stepsof monitoring the interrogation signal frequency and preventing alarmindications when said interrogation signal frequency is outside saidgiven range.
 2. A method for reducing the occurrence of false alarms inan electronic theft detection system of the type wherein aninterrogation signal is varied in frequency and wherein predeterminedchanges in the electromagnetic field in the vicinity of a checkpoint areproduced by the passage therethrough of protected articles carryingspecial electronic circuits whenever said interrogation signal passesthrough a given frequency range, and wherein an alarm indication isproduced in response to said predetermined changes, said methodcomprising the steps of monitoring for similar changes in theelectromagnetic field in the vicinity of said checkpoint which occurwhile said interrogation signal is outside said given range andpreventing alarm indications in response to detection of said similarchanges when said interrogation signal subsequently passes through saidgiven range.
 3. A system for detecting the unauthorized passage ofarticles through a checkpoint, said system comprising a transmitteroperative to transmit an interrogation signal which varies in frequency,responder circuits mounted on articles whose unauthorized passage is tobe detected, said responder circuits being operative to produce apredetermined change in the electromagnetic field conditions in thevicinity of said checkpoint in response to the incidence on saidcircuits of said interrogation signal within a given frequency range,receiver mEans operative in response to said predetermined change toproduce an alarm actuating signal, alarm means responsive to saidactuating signal to produce an alarm indication and means operative toprevent alarm actuation while said interrogation signal is outside saidgiven frequency range.
 4. A system according to claim 3 wherein saidresponder circuits include energy absorbing resonant circuits andwherein said receiver means is connected to respond to energy levelchanges in the vicinity of said checkpoint caused by the absorption ofpower in a responder circuit passing through the checkpoint.
 5. A systemaccording to claim 3 wherein said means operative to prevent alarmactuation comprises a gate circuit associated with said receiver andarranged in the path of said alarm actuating signal, a frequencysensitive switch connected to receive transmitted interrogation signalsand to produce switching signals to open said gate circuit when saidinterrogation signals are within said given frequency range and to closesaid gate circuit where said interrogation signals are outside saidgiven frequency range.
 6. A system for detecting the unauthorizedpassage of articles through a checkpoint, said system comprising atransmitter operative to transmit an interrogation signal which variesin frequency, responder circuits mounted on articles whose unauthorizedpassage is to be detected, said responder circuits being operative toproduce a predetermined change in the electromagnetic field conditionsin the vicinity of said checkpoint in response to the incidence on saidcircuits of said interrogation signal within a given frequency range,receiver means operative in response to said predetermined change toproduce an alarm actuating signal, alarm means responsive to saidactuating signal to produce an alarm indication and means associatedwith said receiver means for producing alarm inhibit signals in responseto said predetermined change which occurs while said interrogationsignal is outside said given frequency range and means responsive to theoccurance of said alarm inhibit signals for inhibiting alarm indicationswhen said interrogation signal subsequently passes through said givenfrequency range.
 7. A system according to claim 6 wherein said respondercircuits include energy absorbing resonant circuits and wherein saidreceiver means is connected to respond to energy level changes in thevicinity of said checkpoint caused by the absorption of power in aresponder circuit passing through the checkpoint.
 8. A system accordingto claim 6 wherein said means associated with said receiver meanscomprises first and second gate circuits each connected to receivesignals from said receiver means, one of said gate circuits beingconnected to permit signals passing therethrough to produce an alarmindication, the other gate circuit being connected to permit signalspassing therethrough to override any opening of said one gate circuitfor a predetermined length of time and a frequency sensitive switchconnected to receive transmitted interrogation signals and to produceswitching signals to open said one gate when said interrogation signalsare within said given frequency range and to open said other gate whensaid interrogation signals are outside said given frequency range.
 9. Asystem according to claim 8 wherein said frequency sensitive switchincludes a multivibrator having two output terminals which arealternately energized and which are connected, respectively, to controlterminals of said first and second gate circuits.
 10. A system accordingto claim 8 wherein a signal sustaining circuit is connected to theoutput of said other gate circuit.
 11. A system according to claim 10wherein said transmitter is operative to vary the frequency of saidinterrogation signal cyclically at a given rate and wherein said signalsustaining circuit has a duration in excess of the cyclic period offrequency variation of said interrogation signal.